174 AMPHIOXUS 



run obliquely downwards and backwards. The parts 

 of the pharyngeal wall between the successive slits 

 are narrow bars — the gill-arches — each of which is 

 strengthened by an axial rod of a homy substance. 

 These arches are of two kinds, arranged alternately ; 

 the axial rods of the second, fourth, etc. arches being 

 forked at their ventral ends, while the alternate rods 

 are unsplit. Bach double gill-slit is originally a 

 single one, but becomes divided in the course of 

 development by the downgrowth of the unsplit bar 

 from its dorsal edge. The split bars may hence be 

 called primary rods, and the unsplit ones secondary 

 rods. Like the myotomes the gill-arches are not 

 in pairs, but alternate on the two sides of the 

 body. 



The successive gill-arches are connected by hori- 

 zontal bars, of which there are usually ' three or 

 more to each slit, so that the pharynx has the 

 character of a open meshwork. The inner surface 

 of the pharynx is lined by columnar flagellate cells, 

 which maintain a continual stream of water through 

 the gill-slits from the pharynx to the atrial cavity, 

 the stream serving to aerate the blood in the vessels 

 of the arches as it passes over them. 



Along the mid-dorsal line of the pharynx is the 

 deep hyperbrancMal groove, lined by a single layer 

 of long columnar ciliated cells. A band of similar 

 cells runs along the mid-ventral wall of the pharynx, 

 and this is folded longitudinally in its hinder part 

 to form the hypobrancMal groove. 

 i. The intestine commences at the hinder end of the 

 pharynx, close to the dorsal surface, and runs straight 

 to the anus. At its commencement it is very narrow : 

 further back it dilates considerably ; and this expanded 

 part, which is sometimes called ' stomach,' is produced 

 into a large pouch-like sac — the liver— which runs 

 forwards some distance along the right side of the 

 pharynx, and ends blindly in front. 



